A known lawn mower of the type noted above comprises a vehicle body including an engine, a transmission case, running wheels, and a steering handle, and a grass cutting unit mounted on a forward portion of the vehicle body. The vehicle body and cutting unit have respective frames interconnected through a pair of longitudinally arranged connecting bolts. The frame of the cutting unit defines plural pairs of connecting bores, so that the height of the cutting unit is adjustable by selecting one of the pairs of connecting bores.
When adjusting the height of the cutting unit in the above structured walking-operator type lawn mower, the cutting unit is completely detached by removing the pair of connecting bolts and then lifted in a horizontal posture by means of a jack to attach the connecting bolts to a different bore pair. This requires a difficult operation for positioning the connecting bores while lifting the heavy cutting unit in a horizontal posture. Consequently, vertical adjustment of the cutting unit is a troublesome and time-consuming task.
In order to solve the above noted problem, the cutting unit may be made vertically adjustable by means of vertically elongated slots defined in the frame of the cutting unit. However, such a structure would have a practical drawback in that the connecting bolts may be loosened such as by vibrations of the vehicle body, whereby the mounting position of the cutting unit tends to be vertically displaced.